In general, the non-linear optical effect indicates a non-linear optical response proportional to the square, cube or higher power of a photoelectric field applied. Known examples of the second-order non-linear optical effect proportional to the square of a photoelectric field applied include second harmonic generation (SHG), optical rectification, photorefractive effect, Pockels effect, parametric amplification, parametric oscillation, light sum frequency mixing, and light difference frequency mixing. Also, examples of the third-order non-linear optical effect proportional to the cube of photoelectric filed applied include third harmonic generation (THG), optical Kerr effect, self-induced refractive index change, and two-photon absorption.
As for the non-linear optical material exhibiting these non-linear optical effects, a large number of inorganic materials have been heretofore found. However, an inorganic material can be very hardly used in practice because a so-called molecular design so as to optimize the desired non-linear optical characteristics or various properties necessary for the production of a device is difficult. On the other hand, an organic compound can realize not only optimization of the desired non-linear optical characteristics by the molecular design but also control of other various properties and therefore, the probability of its practical use is high. Thus, an organic compound is attracting attention as a promising non-linear optical material.
In recent years, among non-linear optical characteristics of the organic compound, third-order non-linear optical effects, particularly, non-resonant two-photon absorption, are being taken notice of. The two-photon absorption is a phenomenon of a compound being excited by simultaneously absorbing two photons. In the case where the two-photon absorption occurs in the energy region having no (linear) absorption band of the compound, this is called non-resonant two-photon absorption. In the following, even when not particularly specified, “two-photon absorption” indicates “non-resonant two-photon absorption”. Also, “simultaneous two-photon absorption” is sometimes simply referred to as “two-photon absorption” by omitting “simultaneous”.
Meanwhile, the non-resonant two-photon absorption efficiency is proportional to the square of a photoelectric field applied (quadratic dependency of two-photon absorption). Therefore, when a two-dimensional plane is irradiated with a laser, two-photon absorption takes place only in the position having a high electric field strength in the central part of the laser spot, and absolutely no two-photon absorption occurs in the portion having a weak electric field strength in the periphery. On the other hand, in a three-dimensional space, two-photon absorption occurs only in the region having a large electric field strength at the focus where the laser rays are converged through a lens, and two-photon absorption does not take place at all in the off-focus region because the electric field strength is weak. Compared with linear absorption where excitation occurs in all positions proportionally to the strength of a photoelectric field applied, in the non-resonant two-photon absorption, excitation occurs only at one point inside the space because of the quadratic dependency and therefore, the spatial resolution is remarkably enhanced.
Usually, in the case of inducing non-resonant two-photon absorption, a short pulsed laser in the near infrared region having a wavelength longer than the wavelength region where the (linear) absorption band of a compound is present, and having no absorption is used in many cases. Thanks to use of near infrared light in a so-called transparent region, the excitation light can reach the inside of a sample without being absorbed or scattered and one point inside the sample can be excited with very high spatial resolution because of the quadratic dependency of non-resonant two-photon absorption.
The present applicant have filed various patent applications relating to a two-photon sensitization-type three-dimensional recording material using a compound capable of inducing non-resonant two-photon absorption. This recording material is a recording material containing at least (1) a two-photon absorption compound (two-photon sensitizer) and (2) a refractive index-modulating material or a fluorescence intensity-modulating material, where (1) efficiently undergoes two-photon absorption and the obtained energy is transferred to (2) by photoexcited electron transfer or energy transfer to change the refractive index or fluorescence intensity of (2), thereby performing the recording. Thanks to use of non-resonant two-photon absorption but not one-photon absorption employed in the process of light absorption of normal optical recording, a recording pit with three-dimensional spatial resolution can be written at an arbitrary position inside of a recording material.
For example, Patent Document 1 discloses a technique using, as (2) a refractive index- or fluorescence intensity-modulating material, a material capable of modulating the refractive index by the color formation of a dye, or a material capable of modulating the fluorescence from non-fluorescence to fluorescence or from fluorescence to non-fluorescence (a material capable of modulating a refractive index or fluorescence by the color formation of a dye or a fluorescent dye). Also, Patent Document 2 discloses a technique using, as (2) a refractive index- or fluorescence intensity-modulating material, a material capable of forming a seed (latent image speck) through very slight color formation of a dye or change of fluorescence and then performing recording and amplification under light irradiation or heating (a refractive index/fluorescence modulation and latent image amplification system; a material that forms a latent image capable of performing refractive index/fluorescence modulation by color formation of a dye). In addition, for example, Patent Document 3 discloses a technique using, as (2) a refractive index-modulating material, a material capable of forming a macromolecular polymer by polymerization and thereby modulating the refractive index (a material that performs refractive index modulation by polymerization). Furthermore, Patent Document 4 discloses a technique using, as a refractive index-modulating material, a material capable of forming a very fine polymerized latent image speck and then driving the polymerization (a refractive index modulation and latent image polymerization system; a material that forms a latent image capable of performing refractive index modulation by polymerization).
In all of these two-photon sensitization-type three-dimensional recording materials described in Patent Documents 1 to 4, a material capable of performing two-photon absorption with light of 700 nm or more is used as (1) the two-photon absorption compound (two-photon sensitizer). However, there are various demands in recent years, and above all, for obtaining a higher recording density, a recording material capable of performing non-resonant two-photon absorption recording by using recording light in the wavelength region shorter than 700 nm so as to form a smaller pit in the recording material is required. The two-photon absorption compound described in Patent Document 5 can perform non-resonant two-photon absorption recording.
Incidentally, in Non-Patent Document 1, it is disclosed that a compound having a structure shown below exhibits non-resonant two-photon absorption properties for light at 450 to 600 nm.

However, there is no description of the humidity/heat resistance of the non-resonant two-photon absorption recording material using a two-photon absorption compound capable of performing non-resonant two-photon absorption recording by using recording light in the wavelength region shorter than 700 nm described in Patent Document above.